The present invention relates generally to fuel-fired heating equipment, and more particularly relates to fuel-fired water heaters.
Conventionally constructed fuel-fired water heaters are typically of a one pass, up-fired configuration in which a fuel burner is disposed at the bottom end of the heater water storage tank beneath a tube sheet to which vertical heat transfer tubes are connected. These tubes extend vertically through the water in the interior of the tank, and are appropriately connected at their upper ends to an external combustion product vent pipe. During water heater firing, the hot combustion products generated by the fuel burner make a single upward pass through the vertical tubes, thereby transferring combustion heat to the tank water, before being discharged into the external vent pipe.
This conventional up-fired water heater construction, though widely accepted and utilized over the years, is subject to a variety of well known problems, limitations and disadvantages. For example, particularly where "hard" water is being heated, it tends to create scaling, and resulting hot spots, on the tube sheet and heat transfer tubes which leads to premature tube sheet and/or tube burnout.
Additionally, the one vertical pass of hot combustion gases through the heat transfer tubes typically results in a relatively low combustion efficiency, leading to relatively high CO and NOx emission levels and correspondingly low fuel efficiency. Moreover, the high concentration of burner input at the lower end of the water heater tank is undesirable because during periods of high water drawdown (usually from the top of the tank) there tends to be a substantial temperature gradient between the water in the top portion of the tank and the water in the bottom portion of the tank. The result of this heat input concentration at the bottom of the tank is that during high water supply periods the water exiting the tank can be considerably cooler than desired--particularly in smaller storage capacity water heaters.
Various types of two-pass fuel fired water heaters have been previously proposed in an attempt to avoid or alleviate these undesirable operational characteristics of conventional single-pass, up-fired water heaters. However, to applicants' knowledge, none of them have proven to be entirely satisfactory in meeting these goals. In view of the foregoing, it is accordingly an object of the present invention to provide a fuel-fired water heater that eliminates or substantially minimizes the above-mentioned problems, limitations and disadvantages associated with conventional fuel-fired water heaters of the type generally described above.